The present invention relates to a battery cooling system and a method of making such a system, and more specifically, to a battery cooling system that uses a heat pipe to remove heat from a central location of a battery cell.
Effective thermal management is critical to maintaining the safety, performance, and operating life of batteries. In lithium-ion batteries, for example, poor temperature control can lead to, among other issues, capacity fade, self-discharge, and thermal runaway. Typically, this thermal management focuses on removing heat that is generated throughout the battery during charging and discharging.
Currently, heat is removed from lithium-ion batteries from the outside of the battery cells. Heat is transferred by conduction to the edge of the cells, and then removed by air cooling, liquid cooling, fins, phase change material, or other methods. The low thermal conductivity inside the battery cell leads, however, to high peak internal temperatures and temperature non-uniformity within the battery cell.
The recognized optimal method for removing heat with minimal thermal resistance is to transfer heat from the location of the heat generation, which, in the case of a battery cell, is throughout the layers of the battery wrap. This has been recognized by studies published in the literature. Modern lithium-ion batteries are fabricated of a number of thin layers, consisting of positive and negative electrodes, positive and negative current collectors, and separator layers. These layers are then wrapped or stacked into either a cylindrical or flat, also known as prismatic, shape. Up until now, integrating a heat removal mechanism into the interior to this cell has been a challenge due to the complex geometry and the coupled electrochemical-thermal physics of these batteries.
One method of removing heat from inside the battery, as opposed to current external cooling methods, is to integrate microchannels into the battery layers. In one proposed design, microchannels would be integrated into the current collector layer or be placed in a separate sheet between a split current collector layer. Passive, two-phase liquid vapor flow through these channels would then cool the battery. Alternatively, it has been proposed to incorporate channels in the positive and negative electrode layers, using water or electrolyte as the working fluids. Studies of both methods have suggested that internal cooling methods can significantly improve battery thermal performance, but incorporating microchannels into either location is a substantial manufacturing challenge.
In addition to the heat removal methods described above, heat pipes have been recognized as a potential method of removing heat from batteries. Heat pipes use passive liquid-vapor phase change to provide very high effective thermal conductivities and transfer heat at a uniform temperature, making them well suited for battery thermal management systems. These approaches, however, typically remove heat from the external surface of the battery cells. That is, the heat pipes are placed between the battery cells and thus the batteries can still suffer from high internal peak temperatures and temperature non-uniformity due to the low thermal conductivity of the battery cells.
An abstract (Shah, K., C. McKee, D. Chalise, A. Jain (2016). Experimental and Numerical Investigation of Heat Pine Based Cooling of Lithium-Ion Cells) was distributed among attendees at the First Pacific Rim Thermal Engineering Conference held between Mar. 13-17, 2016. The abstract authors proposed embedding an open pipe inside a battery cell, and placing a heat pipe inside the open pipe. Reference is made to a test cell, not a battery cell, with an electric heater surrounded by poly-dimethylsiloxane (PDMS), with a pipe in the center of the container. They concluded, however, that incorporating a heat pipe in an actual battery cell would require changes in the manufacturing of the Li-ion cell which may lead to reliability issues that would have to be carefully studied.
The present invention's approach has several differences from that broadly outlined in the above-mentioned abstract that make manufacture in modern battery cell designs practical, including (a) not needing the heat pipe to be placed in a separate hollow tube, (b) not requiring extension through both ends of the cell so as to minimize space and sealing requirements, (c) using a thin electrically-isolating layer, (d) identifying methods of sealing the heat pipe in the battery case, (e) being able to employ multiple battery cell geometries and (f) being capable of simultaneously using internal and external cooling to improve cell thermal performance.
The present invention thus provides a new, improved and practical method of removing heat from individual battery cells using heat pipes. In the present invention, the heat pipe is located centrally in the battery cell without the need for a separate pipe to transfer heat from the internal components to the condenser of the heat pipe located outside of the battery cell. The heat pipe is electrically isolated from the internal battery materials with a thin layer and does not require the heat pipe to have material compatibility with the electrolyte or other battery materials. Another advantage of this technology is that it can be applied to cylindrical or flat cells and be used to remove all of the generated heat or be combined with known simultaneous external cooling methods.
In the present invention, the heat pipe is located in the center of an individual battery cell. The heat pipe can be cylindrical or have a flat profile. A cylindrical heat pipe diameter can range, for example, from 0.1 mm to 150 mm, and a flat heat pipe can, for example, have a thickness of 0.1 mm to 150 mm and width of 0.1 mm to 1000 mm. With either type of heat pipe, the heat pipe evaporator is located centrally of the cell and the condenser is located outside the cell. The condenser can be cooled using an external cooling source such as air cooling, liquid cooling, two-phase cooling, thermal energy storage, or conduction into a cold plate. The surface of the condenser can be enhanced by fins or other structures, and the heat pipe portion outside the battery cell can be straight or curved. The heat pipe of the present invention could also be replaced with similar passive, two-phase flow devices such as thermosiphons or loop heat pipes.
The present invention can be fabricated by directly wrapping the battery layers around the heat pipe, or placing the heat pipe inside a stabilization device such as a pipe or half-pipe to provide structural rigidity. This stabilization device is advantageous because heat pipes are hollow with thin walls, which may not be sufficient to withstand the forces applied during battery fabrication. The stabilization device could be removed after the winding has begun or after fabrication. With prismatic or flat battery geometries, the battery materials can still be wrapped around a flat heat pipe, or alternatively be stacked on each side of a flat heat pipe.
The heat pipe is electrically isolated from the internal battery materials with a thin layer of material, such as epoxy, that has an appropriate low electrical conductivity and good material compatibility with the battery and heat pipe materials. This material can be directly deposited on the surface of the heat pipe or the heat pipe can be placed in a mold which is filled with the material. The material can be applied to the entire heat pipe or just to the portion of the heat pipe located inside the battery cell. This electrical isolation allows the heat pipe to be fabricated from materials that are not chemically compatibility with the electrolyte or other battery materials. In addition, the approach of the present invention allows the heat pipe to be electrically isolated from the positive and negative terminals if desired. This provides flexibility in the design of the module and pack-level cooling strategy.
The evaporator portion of the heat pipe is sealed inside the battery cell, and the remainder of the heat pipe can extend out one or both sides of the battery cell. An O-ring, compression fitting, or weld can be employed where the heat pipe passes through the cell case to seal the case. This sealing can be used with either cylindrical or flat cells, and the cells can have either hard cases or be placed in pouches.
The cooling approach of the present invention can be used as the sole source of heat removal from the battery or simultaneously with external cooling approaches. If used alone, the area surrounding the battery cells can be used to thermally isolate the surrounding cells from each other, thereby improving battery safety in the event of an individual battery cell failure. In many external cooling methods, battery cell failure can propagate to neighboring cells and further damage the battery pack. Alternatively, the simultaneous use of the present invention with an external cooling method can remove larger amounts of heat from the battery cell and improve battery thermal management. The simultaneous external cooling method could, for example, cool the sides of the cell, cool the ends of the cell, or cool the cell via battery bus bars. The external cooling method could also cool the condenser portion of the heat pipe.